Evoked by electrical stimulation of the mental nerve, the masseter inhibitory reflex consists of an early and a late silent period (SP1 and SP2), which interrupt the voluntary electromyographic (EMG) activity in the masseter muscle. We recorded the masseter inhibitory reflex and measured its latency, depth of suppression, duration and recovery cycle to paired stimuli, in patients with Huntington's chorea. Parkinson's disease, dystonia, or unilateral masticatory spasm. In patients with Huntington's chorea the reflex data and recovery cycle were normal. In patients with Parkinson's disease or dystonia, although the reflex data were normal, SP2 recovered far more rapidly than it did in control subjects. This is possibly due to hypoactivity of an inhibitory control of the polysynaptic chain of ponto-medullary interneurons that mediate SP2. In patients with unilateral masticatory spasm, both SP1 and SP2 were absent. Suppression is probably absent because this involuntary movement originates at a point along the peripheral course of the nerve.